Method for self-doping contacts to a semiconductor

ABSTRACT

The present invention provides a system and method for creating self-doping contacts to silicon devices in which the contact metal is coated with a layer of dopant and subjected to high temperature, thereby alloying the silver with the silicon and simultaneously doping the silicon substrate and forming a low-resistance ohmic contact to it. A self-doping negative contact may be formed from unalloyed silver which may be applied to the silicon substrate by either sputtering, screen printing a paste or evaporation. The silver is coated with a layer of dopant. Once applied, the silver, substrate and dopant are heated to a temperature above the Ag—Si eutectic temperature (but below the melting point of silicon). The silver liquefies more than a eutectic proportion of the silicon substrate. The temperature is then decreased towards the eutectic temperature. As the temperature is decreased, the molten silicon reforms through liquid-phase epitaxy and while so doing dopant atoms are incorporated into the re-grown silicon lattice. Once the temperature drops below the silver-silicon eutectic temperature the silicon which has not already been reincorporated into the substrate through epitaxial re-growth forms a solid-phase alloy with the silver. This alloy of silver and silicon is the final contact material, and is composed of eutectic proportions of silicon and silver. Under eutectic proportions there is significantly more silver than silicon in the final contact material, thereby insuring good electrical conductivity of the final contact material.

PRIORITY REFERENCE TO PRIOR APPLICATIONS

This application claims the benefit of U.S. Provisional Application60/167,358, filed Nov. 23, 1999, to Daniel L. Meier, Hubert P. Davis,Ruth A. Garcia and Joyce A. Jessup, entitled “Self-Doping Contacts toSilicon Using Silver Coated with a Dopant Source”, under 35 U.S.C.§119(e), which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to metal contacts to silicon substratesand other semiconductors in which the contact material includes a supplyof dopant atoms, thereby acting as its own dopant source, to facilitatethe formation of a low-resistance ohmic contact between the contactmaterial and the substrate.

2. Description of the Background Art

In a properly designed p-n junction solar cell, the electrons move tothe metal electrode which contacts the n-type silicon, and the holesmove to the metal electrode which contacts the p-type silicon. Thesecontacts are vitally important to the performance of the cell, sinceforcing current across a high resistance silicon/metal interface orthrough a high resistance electrode material robs useful power from thecell. The total specific series resistance of the cell, includinginterfaces and electrode material, should be no more than 1 Ω-cm².

The need for a low-resistance contact places a fairly demandingrequirement on the concentration of dopant atoms at the surface of thesemiconductor. For n-type silicon, this dopant concentration must be≧1×10¹⁹ atoms/cm³ (which is 200 parts per million atomic (ppma) basedupon a density for silicon of 5×10²² atoms/cm³). For p-type silicon therequirement is less severe, with a surface concentration ≧1×10¹⁷atoms/cm³ (2 ppma) being required. Furthermore, to maximize the lightenergy to electrical energy conversion efficiency it is often desirableto have a lower surface doping concentration everywhere on theilluminated side except directly beneath the metal electrode, especiallyfor the n-type surface. Thus, an ideal contact material is one whichsupplies a liberal amount of dopant to the silicon immediately beneathit (also known as self-doping), has a high electrical conductivity,makes a mechanically strong bond to the silicon, and does not degradethe electrical quality of the silicon by introducing sites whereelectrons and holes can be lost by recombination. Finally, this idealcontact material should be inexpensive and should lend itself to beingapplied by an economical process such as screen printing.

A known contact material which possesses, to a significant extent, theabove-described desirable properties, is aluminum. Aluminum possessesthese properties when used for contacting p-type silicon and thereforeforming the positive electrode in a silicon solar cell. This is due tothe fact that aluminum itself is a p-type dopant in silicon. Aluminumcan dope silicon, as part of a process which alloys the aluminum withthe silicon, provided the processing temperature exceeds thealuminum-silicon eutectic temperature of 577° C.

For conventional solar cell structures the lack of a material,comparable to aluminum, for contacting n-type silicon in order to formthe negative electrode of a solar cell, makes the fabrication of asimple, cost-effective solar cell difficult. In a conventional solarcell structure with a p-type base, the negative electrode (whichcontacts the n-type emitter) is typically on the front (illuminated)side of the cell and the positive electrode is on the back side. Inorder to improve the energy conversion efficiency of such a cell, it isdesirable to have heavy doping beneath the metal contact to the n-typesilicon and light doping between these contacts. Thus, the conventionalsilicon solar cell structure presently suffers from a loss ofperformance because of the opposing demands for high doping densitybeneath the contact metal and low doping density between the contactmetal areas.

Existing technology for solar cell contacts to silicon (Si) utilize asilver (Ag) paste with glass frit (e.g., Ferro 3347, manufactured by theElectronic Materials Division of Ferro Corporation, Santa Barbara,Calif.) fired at ≈760° C. The glass frit promotes adhesion of the Aglayer to the Si surface. Such a contact requires a Si substrate whichalready has a heavily-doped surface layer (sheet resistance <45 Ω/□).The interface between the Si and the contact material usually dominatesthe series resistance of the entire cell. Thus, this technology alsoforces the cell designer to create a surface layer which is moreheavily-doped than desired in order to bring the interface resistance toan acceptable level.

Therefore, what is needed is a method and apparatus for self dopingcontacts to a semiconductor, said contacts being heavily doped beneaththe bonding point to the semiconductor but lightly doped between thecontacts, having high electrical conductivity, and a strong mechanicalbond which is easily fabricated and cost effective.

SUMMARY

The present invention provides a system and method for creatingself-doping contacts to silicon devices in which the contact metal iscoated with a layer of dopant, alloyed with silicon and subjected tohigh temperature, thereby simultaneously doping the silicon substrateand forming a low-resistance ohmic contact to it.

A self-doping negative contact may be formed from unalloyed Ag which maybe applied to the silicon substrate by either sputtering, screenprinting a paste or evaporation. The Ag is coated with a layer ofdopant. Once applied, the Ag, substrate and dopant are heated to atemperature above the Ag—Si eutectic temperature (but below the meltingpoint of Si). The Ag liquefies more than a eutectic proportion of thesilicon substrate. The temperature is then decreased towards theeutectic temperature. As the temperature is decreased, the moltensilicon reforms through liquid-phase epitaxy and while so doing dopantatoms are incorporated into the re-grown lattice.

Once the temperature drops below the silver-silicon eutectic temperaturethe silicon which has not already been reincorporated into the substratethrough epitaxial re-growth forms a solid-phase alloy with the silver.This alloy of silver and silicon is the final contact material, and iscomposed of eutectic proportions of silicon and silver. Under eutecticproportions there is significantly more silver than silicon in the finalcontact material, thereby insuring good electrical conductivity of thefinal contact material.

One possible advantage of the self-doping contact includes theelimination of the need for a pre-existing heavily-doped layer, therebyreducing the number of processing steps. The elimination of theheavily-doped layer also permits the use of a more lightly-doped emitterthan is possible for existing technology. This increases cell efficiencybecause of the resulting higher cell photocurrent. Furthermore, adhesionof the contact to the Si surface may be improved over existingtechnology by specifying that alloying occur between Ag and Si. Analloyed contact is more adherent than a deposited contact, even if thedeposited contact has glass frit. In addition, it has been demonstratedthat an alloyed 146A contact remains intact after dipping in HF, unlikea deposited contact with glass frit which is dislodged from the Sisubstrate by immersion in HF. Such insensitivity to HF for alloyedcontacts opens processing options not available with deposited contacts.

Other possible advantages of the invention will be set forth, in part,in the description that follows and, in part, will be understood bythose skilled in the art from the description or may be learned bypractice of the invention. The advantages of the invention will berealized and attained by means of the elements and combinationsparticularly pointed out in the appended claims and equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a sectional view of a Si substrate with an Ag surfacecoated with liquid dopant;

FIG. 1B shows a sectional view of the substrate of FIG. 1A afteralloying, showing the formation of a heavily doped Si layer;

FIG. 2A shows a sectional view of an n-type Si substrate with phosphorusas the n-type dopant source and aluminum as the p-type dopant source;

FIG. 2B shows a sectional view of a p-n junction diode with self-dopingcontacts formed from the structure of FIG. 2A according to an embodimentof the present invention;

FIG. 3A shows a sectional view of an n-type Si substrate with phosphorusas the n-type dopant source and boron as the p-type dopant source;

FIG. 3B shows a sectional view of a p-n junction diode with self-dopingcontacts formed from the structure of FIG. 3A according to an embodimentof the present invention;

FIG. 4 shows a cross-sectional view of a silver particle coated withliquid dopant, where such a coated particle is suitable forincorporation into a screen-printing paste;

FIG. 5 shows a silver-silicon phase diagram which is utilized inaccordance with the present invention;

FIG. 6 shows a current versus voltage plot of a Ag/np+/Al samplestructure after 800 degrees C., two minute heat treatment;

FIG. 7 shows a current versus voltage plot of a Ag/n+np+/Al samplestructure after 900 degrees C., two minute heat treatment;

FIG. 8 shows a current versus voltage plot of a Ag/n+nn+/Ag resistorstructure obtained with phosphorus dopant on both Ag surfaces, processedat 900 degrees C. for two minutes;

FIG. 9 shows a current versus voltage plot of a Ag/n+np+/Ag diodestructure obtained with phosphorus dopant on one Ag surface and borondopant on the other Ag surface, processed at 900 degrees C. for twominutes;

FIG. 10 shows a phosphorus and silver depth profile of a sample afterremoval of the front silver surface, alloyed at 1000 degrees C. for twominutes;

FIG. 11 shows a current versus voltage plot of a fully metallizedresistor structure with self-doping contacts formed according to anembodiment of the present invention; and

FIG. 12 shows a current versus voltage plot of a fully metallized diodestructure with self-doping contacts formed according to an embodiment ofthe present invention.

FIG. 13 shows a current versus voltage plot, measured under anillumination level of 100 mW/cm², of a fully metallized solar cell withself-doping contacts formed according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe embodiments will be readily apparent to those skilled in the art,and the generic principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the invention. Thus, the present invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles, features and teachings disclosed herein.

One approach to producing self-doping contacts uses a combination ofmaterials and processing conditions which produces a self-dopingnegative electrode for silicon solar cells, similar in function to thewidely-used aluminum self-doping positive electrode. Experimentalresults have shown that a combination of antimony as the n-type dopantand silver as the primary contact metal satisfies the basic requirementsfor a self-doping negative electrode. Alternatively, analogousself-doping positive electrodes have been proposed using gallium andsilver. This approach requires that the contact material be applied tosilicon as an alloy of silver and a dopant, such as silver-antimony orsilver-gallium. Current technology for producing small (3 micron) silverparticles for incorporation into a screen-printing, paste utilize theprecipitation of silver particles from a solution of silver nitrate, andis not suitable for producing particles composed of silver and a dopantin alloy form. Another approach introduces dopant atoms to the processseparately from a remote source, generally a gas, during the heatingprocess. Such approaches are described in U.S. patent application Ser.No. 09/072,411 to Meier and Davis, dated May 4, 1998, hereinincorporated by reference.

An embodiment of the present invention which does not require theapplication of an alloy of silver and a dopant or a remote doping sourceis illustrated in FIGS. 1A and 1B. FIG. 1A shows a sectional view of astarting structure comprising a silicon (Si) substrate 102 contacted bya layer of silver (Ag) 104 which, in turn, is coated with a layercontaining a dopant 106. Ag is a widely-used contact metal because ofits low electrical resistivity and solderability. The dopant layer 106may be applied using a commercially-available liquid source.Alternatively, the Ag layer 104 and dopant layer 106 may be applied bysputtering, screen printing or evaporation.

If the temperature of this structure is raised above the Ag—Si eutectictemperature (>835° C.), Ag can alloy with Si to form a liquid poolcontaining Ag, Si, and the dopant. As shown in FIG. 1B, while cooling to835° C. the Si re-grows by liquid phase epitaxy and incorporates dopantatoms into the epitaxial Si layer 112. When the temperature drops below835° C., the liquid pool solidifies abruptly into a two-phase eutecticregion 118; a Si phase 114 which also contains dopant, and a Ag phase116 which is electrically conductive and contains some dopant as well,the two phases being in intimate contact.

A preferred conductive metal for this invention is silver. In additionto its high electrical conductivity, silver has the desirable propertythat its oxide is unstable at temperatures only modestly elevated aboveroom temperature. This means that the alloying process described willyield a contact with an oxide-free surface, even if the alloying is donein air or in oxygen. The oxide-free silver contact is very well suitedfor soldering when cells are interconnected to form a module. Inaddition, the formation of a self-doping negative electrode at atemperature in the range of 835° C. to 1000° C. means its formation canbe combined with the creation of a thermal oxide layer grown on theexposed silicon substrate. This oxide layer would serve to passivate thesilicon surface, thereby reducing the loss of photogenerated electronsand holes by recombination at the surface.

This concept can now be applied to create a complete p-n junction diodefrom an n-type Si substrate in a single high-temperature step (>835°C.). FIG. 2A shows a sectional view of an n-type Si substrate 202 withan Ag layer 204 coated with a liquid phosphorus (P) layer 206 as then-type dopant source and an aluminum (Al) layer 208 as the p-type dopantsource. FIG. 2B shows a cross sectional view of the substrate of FIG. 2Aafter high temperature alloying. Al is used to form the p⁺ region 212 aswell as ohmic contact to that p⁺ region, while Ag coated with P is usedto form the n⁺ region 214 and ohmic contact to it. The contact metalsare Al—Si eutectic 216 and Ag—Si eutectic 218, respectively. The finalAg/n⁺np⁺/Al structure 201 constitutes a complete p-n junction diode withself doping contacts. Note that no separate dopant diffusion step isneeded in this process. Dopant to create the n⁺ and p⁺ regions issupplied either directly by the Al or indirectly by the P coating on theAg via the metal layers.

A second embodiment of the present invention is illustrated in FIGS. 3Aand 3B. FIG. 3A shows a sectional view of an n-type Si startingsubstrate 302 with a first Ag layer 304 coated with a liquid P layer 306as the n-type dopant source and a second Ag layer 308 coated with aliquid Boron (B) layer 310 as the p-type dopant source. FIG. 3B shows across sectional view of the substrate of FIG. 3A after high temperaturealloying. Analogous to the first embodiment, B is used to form the p⁺region 312 as well as ohmic contact to that p⁺ region, while Ag coatedwith P is used to form the n⁺ region 314 and ohmic contact to it. Thefinal Ag/n⁺np⁺/Ag structure 301 constitutes a complete p-n junctiondiode with first solderable Ag contacts 316 and second solderablecontacts 318. Solderability follows from the fact that the oxide of Agis volatile above room temperature, so that a clean Ag surface ispresent after alloying at high temperature.

A third embodiment of the present invention, shown in FIG. 4, combinestwo existing materials, Ag in particle form and a dopant in liquid form,to create a self-doping, screen printable paste. Rather than coat aplanar Ag surface with dopant, as illustrated previously, the entireouter surface of an individual Ag particle 402 is coated with a dopantlayer 404. These coated Ag particles 401 can then be introduced into apaste formulation with binders, solvents, etc., to make ascreen-printing paste (not shown). Silver pastes, usually with glassfrit, are widely used in the photovoltaic industry. Therefore, a dopantmaterial which can be applied as a coating to Ag can generally functionas a dopant source in the alloying process. This includes a variety ofcommercially-available liquid dopants such as P, antimony (Sb), arsenic(As), indium (In), aluminum (Al) and gallium (Ga). A coating ofelemental Sb, Al, Ga, or In on the Ag particles may also serve as adopant source. Since it is not uncommon for manufacturers ofscreen-printing pastes to coat Ag particles with a layer of material toprevent agglomeration of the small particles, the technology forapplying a coating to Ag particles already exists for some materials.This embodiment of the invention in which each Ag particle in the pasteis coated with liquid dopant can be applied to make screen-printingpaste.

A silver-silicon phase diagram for this method is shown in FIG. 5. Thevertical axis of FIG. 5 is temperature in degrees centigrade, while thehorizontal axis is percentage silver. The horizontal axis has twoscales: a lower scale of percent silver (by weight) and an upper scaleof percent silver (atomic). A eutectic point 502 is found at 96.9% Agand 3.1% Si (by weight). Eutectic point 502 lies on line 504 whichindicates a temperature of 835° C. Also shown are the melting point 506of Ag (961.93° C.) and the melting point 508 of Si (1414° C.). Curve 510(which rises leftward from point 502) indicates that as the temperatureis further increased above the eutectic, the percent Si, which can beheld in a molten mixture of Si and Ag, also increases. Silver istherefore capable of dissolving silicon at temperatures above 835° C.,and then allowing the silicon to recrystallize by liquid phase epitaxyupon cooling, in analogy with the behavior of aluminum. Unlike aluminum,however, silver is not a dopant in silicon, so a dopant, some of whichwill remain in the silicon upon epitaxial re-growth, must be added tothe silver. From the phase diagram it can be seen that the eutecticmaterial will have two regions (phases), a major region which is nearlypure Ag and a minor region which is nearly pure Si.

The phase diagram of FIG. 5 also gives a way of determining the amountof silicon that a given thickness of silver will dissolve. It therebyprovides a means for estimating eutectic layer thickness and n⁺njunction depth for the case where Ag is in contact with an n-typesubstrate and is coated with an n-type dopant. The ratio of thickness ofsilicon dissolved (t_(Si)) to thickness of silver deposited (t_(Ag)) atan alloying temperature,(T) is given by:

(t _(Si))/(t _(Ag))=(ρ_(Ag))/(ρ_(Si))*[W _(Si)(T)/(100%−W_(Si)(T))]  (1)

where ρ_(Ag) is the density of silver (10.5 g/cm³), ρ_(Si) is thedensity of silicon (2.33 g/cm³), and W_(Si)(T) is the weight percent ofsilicon at the processing temperature. With W_(Si)(T=835° C.) of 3.1%from the phase diagram, the thickness ratio is calculated from Equation(1) to be 0.144. Thus, the Ag—Si eutectic layer will be 1.144 times asthick as the Ag layer.

The depth of the n⁺n junction that would be found beneath the Ag regionof the eutectic layer depends on the temperature at which the alloyingwas done, as indicated by Equation (1). (The n⁺ region is theheavily-doped epitaxial layer 112 in FIG. 1B.) For example, at 900° C.,W_(Si) is 4.0% (from the left liquidus branch of the phase diagrambecause excess Si is available for the limited Ag to dissolve) andt_(Si)/t_(Ag) is 0.188, while at 1000° C., W_(Si) is 5.8% andt_(Si)/t_(Ag) is 0.278. The depth of the junction beneath the Ag regionfor a contact alloyed at temperature T is then given by:

x _(j)(T)=Δt _(Si)(T)={[t _(Si) /t _(Ag)](T)−[t _(Si) /t _(Ag)](T_(eutectic))}* t _(Ag)  (2)

Equation (2) shows that x_(j)(T=900° C.) is 0.044*t_(Ag) andx_(j)(T=1000° C.) is 0.134*t_(Ag). For example, a 10 μm thick Ag layerwill dissolve 1.88 μm of Si at 900° C. and create a junction depth of0.44 μm upon epitaxial re-growth, while at 1000° C. a 10 μm thick Aglayer will dissolve 2.78 μm of Si and create a junction depth of 1.34μm.

It is noteworthy that semiconductors other than silicon interact withsilver in a similar way. In particular, the binary phase diagram ofgermanium with silver exhibits a eutectic at 650 C having composition81% silver and 19% germanium by weight. The melting point of germaniumis 937 C. Like silicon, germanium is a member of Group IV of theperiodic table so that elements from Group III and Group V act as p-typeand n-type dopants in germanium, respectively. Germanium alsocrystallizes in the diamond cubic structure, like silicon. This meansthat the concept of a self-doping contact, as described above forsilicon and silver, can be extended to germanium and to semiconductoralloys of silicon and germanium.

EXPERIMENTAL RESULTS

Embodiments of this invention have been tested experimentally withsilicon using both evaporated Ag layers and screen-printed Ag layers,and the key features of the self-doping alloyed Ag contacts have beendemonstrated. Diodes and resistors were made using evaporated Ag alongwith liquid P and B dopants. Electrical measurements, includingcurrent-voltage (I-V) curves and spreading resistance profiles, as wellas examinations by scanning electron microscopy (SEM), scanning Augermicroanalysis (SAM), and secondary ion mass spectroscopy (SIMS)confirmed the creation of a self-doping contact when the processingtemperature exceeded the eutectic temperature. Contact (interface)resistance and bulk metal resistivity were consistent with an effectiveohmic contact. In addition, an experimental Ag paste has been formulatedwhere the individual Ag particles have a coating which acts as a sourceof P dopant. Optical microscopy has shown that this paste gives rise toAg—Si alloying. I-V curves for resistors and diodes, type-testing, andmeasurements of contact resistance and spreading resistance all showthat this paste is self-doping. Solar cell grid patterns, and prototypedendritic web solar cells have also been made using this paste. Such apaste is desirable as a cost-effective means of implementing self-dopingcontacts in solar cells.

Samples were prepared using dendritic web silicon substrates, 2.5cm×10.0 cm in area, approximately 120 μm thick, and doped n-type (Sb) toapproximately 20 Ω-cm. A layer of Ag, 2-4 μm thick, was evaporated onone side of the substrate and a layer of Al, 2-4 μm thick, wasevaporated on the opposite side. A coating of Filmtronics P507 liquidphosphorus dopant from Filmtronics Semiconductor Process Materials ofButler, Pa., was painted onto the Ag surface in most cases and thendried. Heat treatment was done in a Modular Process Technology (MPT)model 600S rapid thermal processing (RTP) unit at temperatures rangingfrom 800° C. to 1000° C., typically for 2 minutes in flowing argon (Ar)gas. With this temperature range, the Al—Si eutectic temperature (577°C.) was always exceeded, so the Al always gave rise to a p⁺ layer.However, the Ag—Si eutectic temperature (835° C.) was exceeded in somecases and not in others. In addition, some test structures wereprocessed over the same temperature range with no P507 phosphorus dopantlayer applied to the Ag surface.

In a first experiment, a starting structure comprising an n-type Sisubstrate contacted by a layer of Ag was, in turn, coated with a layercontaining a dopant. The structure was subjected to 900° C., 2 minute,RTP heat treatment, and then cooled. Under SEM inspection (without theAg layer removed), two distinct regions on the Ag surface were clearlyevident as expected from the phase diagram of FIG. 5 and the schematicof the Ag—Si eutectic layer 118 of FIG. 1B. Auger spectroscopy withdepth profiling was used to show that darker regions were Si and lighterregions were Ag. Symmetrical patterns reflected the surface orientationof the Si web substrate. Thus, alloying of Ag and Si occurred, asexpected, since the processing temperature (900° C.) exceeded theeutectic temperature (835° C.). Several small particles (approximately1-2 μm) of Ag were also present on the surface.

In a second experiment, a 2.5 cm×10.0 cm P507/Ag/n-Si/Al substratestructure was processed at 800° for 2 minutes. After cooling, a 2.0cm×2.0 cm sample was cut from the structure and electrically tested. Theresultant I-V curve 602, represented in FIG. 6, indicated only very highresistance exceeding 1 kΩ-cm². Such a high resistance is a consequenceof the failure of the Ag and Si to alloy, since the processingtemperature was below the eutectic temperature. Consequently, there wasno liquid region formed and no way for the dopant to become incorporatedinto the surface of the Si. This was confirmed by examining the Sisurface under SEM inspection after the Ag was removed by etching. Thesurface was featureless, indicating no alloying and no self-dopingaction. Only a highly-resistive Ag/np⁺/Al structure was created.

In a third experiment, a 2.5 cm×10.0 cm P507/Ag/n-Si/Al substratestructure was processed at 900° for 2 minutes. After cooling, a 2.0cm×2.0 cm sample was cut from the structure and electrically tested. Theresultant I-V curve 702, represented in FIG. 7, indicated the formationof a self-doping Ag/n⁺np⁺/Al structure, and the creation of atextbook-like Si diode. The low-leakage p⁺n junction is an Al alloyjunction. The low resistance ohmic contact to the n-type substratefollows from the alloying action of Ag, in conjunction with a P dopantsource, to create the n⁺ layer. After the Ag was removed by etching, SEMinspection revealed that the Si surface exhibited a distinct topographyassociated with the formation of the Ag—Si eutectic. As represented inFIG. 1B, the Si columns 114 were raised approximately 2 μm above thefloor of the silicon 112 which had been covered with the Ag portions ofthe eutectic layer 118 prior to Ag etching. A measurement of the sheetresistance of the front Si n⁺ surfaces gave 70 Ω/□ for the surface.Other measurements showed that the resistivity of the Ag—Si eutecticcontact metal is 2.0 times as high as the resistivity of the evaporatedAg. However, the resistivity of the eutectic metal is still quite low at≈6 μΩ-cm, considering the handbook value of resistivity for bulk Ag is1.6 μΩ-cm.

In a fourth experiment, a 2.5 cm×10.0 cm P507/Ag/n-Si/Al substratestructure was processed at 1000° for 2 minutes. After cooling, a 2.0cm×2.0 cm sample was cut from the structure and electrically tested. Theresultant I-V curve was essentially identical to curve 702 representedin FIG. 7, and again indicated the formation of a self-dopingAg/n⁺np⁺/Al structure, and the creation of a textbook-like Si diode. Ameasurement of the sheet resistance of the front Si n⁺ surfaces gave 40Ω/□ for the surface. Other measurements showed that the resistivity ofthe Ag—Si eutectic contact metal is 2.2 times as high as the resistivityof the evaporated Ag. In both the third and fourth experiments, thestructure 201 of FIG. 2B was therefore realized in practice.

An estimate of the I-V curve that might result if this process wereapplied to a dendritic web silicon solar cell structure can be made bytranslating the I-V curve 702 of FIG. 7 downward along the current axisby 120 mA (typical J_(SC) value of 30 mA/cm²). Such an estimate givesV_(OC) of 0.57 V, Fill Factor (FF) of 0.78, and efficiency (η) of 13%.The sharp knee 704 of the diode I-V curve 702 (as also indicated by thehigh estimated FF), the high estimated V_(OC), and the low reverse biasleakage current all suggest that Ag is not contaminating the Sisubstrate or the p-n junction at 900° C. or 1000° C. The implication isthat high efficiency solar cells can be made with this contact system.This was later confirmed when complete solar cells were fabricated usinga self-doping silver paste, as shown in FIG. 13.

Additional information regarding contact resistance was obtained byevaporating approximately 2 μm Ag on both sides of an n-type websubstrate and applying phosphorus liquid dopant to both Ag surfaces fora starting structure of P507/Ag/n-Si/Ag/P507. After RTP alloying at 900°C. for 2 minutes, the linear I-V curve 802 of FIG. 8 was obtained,indicating the formation of a resistor with a Ag/n⁺nn⁺/Ag structure.From the slope of the I-V curve 802, a specific resistance of 0.12 Ω-cm²is obtained. This can be attributed entirely to the resistance of thesilicon substrate, indicating a negligible contact resistance associatedwith the Ag metal and Ag/Si interface.

To illustrate the versatility of the Ag-based self-doping contactsystem, phosphorus liquid dopant was applied to one Ag surface and acommercial boron liquid dopant (Boron-A) from Filmtronics was applied tothe other Ag surface to give a starting structure ofP507/Ag/n-Si/Ag/Boron-A. After RTP alloying at 900° C. for 2 minutes,the rectifying I-V curve 902 of FIG. 9 was obtained, indicating theformation of a Ag/n⁺np⁺/Ag structure in one high-temperature step. Inthis case the p-n junction was formed by alloying Ag with Si in thepresence of B dopant, while ohmic contacts followed from the creation ofthe n⁺ and p⁺ layers in intimate contact with the Ag—Si eutectic layer.The structure 301 of FIG. 3B was therefore realized in practice.

Measurements of I-V curves and sheet resistance indicated P had beenincorporated into the Si to form an n⁺ layer during alloying, but didnot detect P directly. SIMS was employed to determine the composition-ofthe surface layer for samples taken in the second, third and fourthexperiments after the front Ag was removed. Data showed doping of2×10²⁰P/cm³ to a depth of 0.3 μm at 900° C., 2×10²⁰P/cm³ to a depth of0.4 μm at 1000° C., and no appreciable P-doping at 800° C., in goodagreement with diode I-V curves. This shows that a necessary conditionfor the formation of a self-doping contact is that Ag alloy with Si,i.e., that the processing temperature exceed the eutectic temperature of835° C. As seen in FIG. 10, Ag appeared to be below the detection limit(<1×10¹⁵Ag/cm³) at depths greater than 1 μm, suggesting that Ag will notcontaminate the Si in the alloying process (in agreement with the diodeI-V curve 702 of FIG. 7). The SIMS depth profile 1002 for P and depthprofile 1004 for Ag for the sample alloyed at 1000° C. for 2 minutes isshown in FIG. 10, where an n⁺n junction depth of 0.4 μm is indicated.From Equation (2) this depth implies a starting Ag thickness of 3.0 μm,which is consistent with the estimated thickness of evaporated Ag of 2-4μm. The gradual reduction in measured P and Ag concentrations from 0.4μm to 1.0 μm in FIG. 10 may be associated with the Si columns in theeutectic layer which are presumed to contain P and Ag. The overall Pconcentrations and junction depths obtained by SIMS are in reasonableagreement with those obtained by spreading resistance measurements.

Some Ag/n-Si/Al samples were prepared with no dopant coating on the Aglayers. After processing under the same conditions described previously(temperatures up to 1000° C.), I-V curves showed extremely high seriesresistance. This demonstrates that self-doping action does not occurbecause of the Ag itself, but only if a dopant coating is applied to theAg surface. These experiments suggest that conditions for achieving aself-doping Ag contact to Si are:

1. Coating the Ag surface with a dopant source;

2. Using a processing temperature which exceeds the Ag—Si eutectictemperature so that alloying of Ag with Si occurs.

The self-doping alloyed Ag contact system has also been implemented in ascreen-printing paste. DuPont Electronic Materials, Research TrianglePark, NC, has formulated an experimental paste in response to a requestand specification from EBARA Solar. This paste is designated by DuPontas E89372-146A, and contains Ag particles which are coated with a layerwhich contains P.

The ability of the 146A paste to create a self-doping contact wasdemonstrated by converting the surface of a p-type dendritic web siliconsubstrate to n-type. A p-n junction diode (sample 146A-1000p) wasfabricated with a low-resistivity p-type web (0.36 Ω-cm) serving as thestarting substrate. A back ohmic contact was made by alloying FerroFX-53-048 Al paste to make a pp⁺ structure. DuPont 146A paste was thenprinted over nearly the entire front of the blank (2.5 cm×10.0 cm) anddried (200° C., 10 minutes, Glo-Quartz belt furnace). Binder burnout andAg alloying were done in the MPT RTP, with alloying at 1000° C. for 2minutes in Ar. A 2 cm×2 cm piece was cut from the blank. The measuredI-V curve was rectifying, with a shunt resistance of 1.6 kΩ-cm², a softturn-on voltage of ≈0.5 V and series resistance in the forward direction<0.94 Ω-cm². The creation of a diode on a p-type substrate indicates ann⁺ layer was formed beneath the 146A metal, as desired, to give aAg/n⁺pp⁺/Al structure. This was confirmed by removing the Ag metal inHNO₃. The underlying Si was found to be strongly n-type by a hot probetype tester, and the sheet resistance was measured in the range 4-28Ω/□. Thus, the front Si structure was confirmed to be n⁺p, with 146Apaste supplying the n-type dopant. For comparison, another p-type webblank was printed with DuPont E89372-119A Ag paste, which is similar tothe 146A paste but without the phosphorus-containing coating, andalloyed as above. Upon stripping the Ag from the front, the underlyingSi tested p-type, as expected, since the 119A has no source of P. Thesupposed structure then is Ag/pp⁺/Al for the 119A paste which is notself-doping. This confirmed that the structure 401 of FIG. 4 wasrealized in practice with the 146A paste.

Additional work with the 146A paste further confirmed its ability toserve as a self-doping contact material. A fully metallized Ag/n⁺nn⁺/Agresistor and a Ag/n⁺np⁺/Al diode were fabricated using n-type websilicon cell blanks (2.5 cm×10.0 cm) in one high temperature step (900°C., 2 minutes, 1 slpm Ar) in the MPT RTP. The source of Al for the diodewas the commercial Ferro FX-53-048 Al paste. Alloying of Ag with Si wasuniform, with only small balls of metal appearing on the surface and nounalloyed areas. Good ohmic contact was obtained for the resistor (0.12Ω-cm², including 0.07 Ω-cm² resistance of bulk Si) as shown in FIG. 11.The linear I-V curve 1102 demonstrates ohmic contact to the 7 Ω-cmn-type dendritic web Si substrate. Total resistance of 0.12 Ω-cm²includes 0.07 Ω-cm² associated with the Si (nominal thickness of 100μm), leaving an estimated net Ag/n⁺ contact resistance of 25 mΩ-cm².

Turning to FIG. 12, the Ag/n⁺np⁺/Al diode was also shown to have verylow leakage current as indicated by its high shunt resistance. Dendriticweb Si substrate was nominally 100 μm thick and had a resistivity of 7Ω-cm. Note the low leakage current (as represented by curve 1202) andthe sharp knee 1204 of the curve.

The ability to print and alloy patterns using the 146A Ag paste was alsodemonstrated. A solar cell grid pattern with Ag lines having a nominal100 μm width was printed and alloyed, along with a contact resistancetest pattern utilizing the current transfer length method (TLM)comprising a series of bars 1 mm wide and 25 mm long. The contactresistance test pattern was printed on 6.8 Ω-cm n-web (no diffusedlayer) and fired at 950° C. in the MPT RTP. This gave uniform, adherentcontacts which showed evidence of Ag—Si alloying (triangles reflectingthe web silicon surface, apparent two-phase region at the surface), andmeasured contact resistance of 2.8 mΩ-cm² for the 146A paste. It wasfurther determined that phosphorus from the Ag was doping the Si beneaththe metal by stripping the metal and probing the Si surface using thespreading resistance technique. Measured spreading resistance decreasedby a factor of 1000 when the probes passed from the region beside the Agbar (6.8 Ω-cm) to the region originally beneath the Ag bar. This impliesa surface concentration of 8×10¹⁸P/cm³ supplied by the 146A paste.Simultaneous type testing also confirmed that both the substrate and theregion beneath the metal were n-type.

The bulk resistivity of the screen-printed and alloyed 146A paste hasbeen measured to be 5 Ω-cm, which is sufficiently low and not muchgreater than the 1.6 μΩ-cm value for pure Ag. Tabs used forinterconnecting cells in a module have also been soldered to the alloyed146A surface. Thus, electrical conductivity and solderability of the146A paste have been demonstrated.

The DuPont 146A fritless, self-doping paste was used to form thenegative contact to PhosTop web solar cells with an Ag/n⁺pp⁺/Alstructure, and having n⁺ sheet resistances of 35 Ω/□ and 70 Ω/□ (LotPhosTop-46). Alloying of 146A Ag was done in the MPT RTP at 900° C. for4 minutes. Results are tabulated in Table 1 below for cells fabricatedwithout an anti-reflective (AR) coating. Commercial Ferro 3347 frittedAg paste, fired in a belt furnace at 730° C., is included forcomparison. In all cases Al alloying in a belt furnace at 850° C.followed the P diffusion and preceded the Ag alloying or firing. At 35Ω/□, where the Si surface is pre-doped liberally with P, cell efficiencyfor the self-doping 146A Ag is comparable to, but no better than, thatfor the 3347 Ag. However, at 70 Ω/□ the 146A gives considerably betterefficiency than does the 3347. The reason for this is that the seriesresistance is quite high (approximately 20 Ω-cm²) for 3347 because of aninsufficient concentration of P at the Si surface, but is at anacceptable level for 146A which supplies its own P. These results showthat the 146A Ag paste enables the use of a more lightly-doped P layer,which is expected to lead to higher efficiency cells when the Si surfaceis properly passivated. It is also expected that an alloying processwhich can be executed in a belt furnace rather than an RTP can bedeveloped for 146A to achieve higher throughput and lower cost.

TABLE 1 J_(sc) Ag R_(sheet) # (no AR) V_(oc) Fill Factor EfficiencyPaste (Ω/□) Cells (mA/cm²) (V) (FF) (%) Ferro 35 30 20.2 ± 0.4 0.578 ±0.758 ± 8.86 ± 0.35 3347 (20.7 best) 0.006 0.016 (9.24 best) (0.583(0.779 best) best) Du- 35 10 18.6 ± 1.9 0.557 ± 0.724 ± 7.49 ± 1.01 Pont(20.5 best) 0.017 0.048 (9.04 best) 146A (0.587 (0.767 best) best) Ferro70 19 13.4 ± 1.8 0.527 ± 0.417 ± 3.02 ± 0.99 3347 (16.4 best) 0.0090.091 (4.75 best) (0.554 (0.596 best) best) Du- 70 10 19.0 ± 0.3 0.557 ±0.728 ± 7.69 ± 0.23 Pont (19.5 best) 0.005 0.011 (8.09 best) 146A (0.563(0.743 best) best)

Additional work was done in which self-doping Ag pastes were alloyedafter P diffusion, but before Al alloying. DuPont 146A fritless Ag pasteas well as DuPont 151B fritted Ag paste were used. Ag particles in the151B paste were identical to those in the 146A paste in that aphosphorus-containing coating had been applied to them, but glass frithad been added to the coated paste so that 151B was a fritted version ofthe 146A Ag paste. The best results obtained when Ag alloying (900° C.for 4 minutes in the MPT RTP for 146A or 940° C. for approximately 1minute in a belt furnace for 151B) preceded Al alloying (800° C. forapproximately 3 minutes in a belt furnace in both cases) are summarizedin the Table 2 below.

TABLE 2 Fill Effi- R_(sheet) PhosTop J_(sc)(no AR) V_(OC) Factor ciencyAg Paste (Ω/□) Lot # (mA/cm²) (V) (FF) % DuPont 57 56 20.2 0.578 0.7528.76 146A DuPont 69 56 20.2 0.584 0.733 8.65 146A DuPont 78 58 18.60.577 0.753 8.09 151B

These data show that good fill factors and other solar cell parameterscan be obtained when the self-doping Ag pastes are applied to a Sisurface doped lightly (approximately 70 Ω/□) with P. An estimate of theefficiency expected if the three cells in the above table had an ARcoating can be obtained by multiplying the observed efficiency (no AR)by 1.45. This gives 12.7%, 12.5%, and 11.7%, respectively, and confirmsthe expectations of FIG. 7 that screen-printed self-doping Ag contactscan be used for solar cells. Furthermore, the fact that the 151B frittedAg paste can be alloyed in a belt furnace shows that such a paste iscompatible with a practical, high-throughput process for formingcontacts. The fabrication of cells made with the 151B Ag paste wasaccomplished by screen-printing (P, Ag, and Al) along with belt furnaceP diffusion (870° C.), Ag alloying (940° C.) and Al alloying (800° C.).

Finally, dendritic web silicon solar cells having self-doping silvercontacts were fabricated, complete with an anti-reflective (AR) coating.The AR coating was silicon nitride (nominal 86 nm thickness and 1.98index of refraction), deposited by plasma-enhanced chemical vapordeposition (PECVD) onto the front n⁺ silicon surface. This was followedby screen-printing and alloying aluminum to form the p⁺n junction andback contact. The structure to which DuPont experimental 151A or 151Bfritted self-doping Ag paste was applied was: SiN_(x)/n⁺pp⁺/Al. Byvirtue of the glass frit, these pastes were able to penetrate throughthe insulating silicon nitride layer to make ohmic contact to n⁺ layershaving sheet resistances up to 100 Ω/□. This high-throughput process wascarried out in a radiantly heated belt furnace at 940 C for 1 minute.The. illuminated I-V curve 1302 for such a cell (Lot PhosTop-69, cell111, 151A paste) is given in FIG. 13. Cell area is 25 cm², and doping ofthe n⁺ layer is very light at 100 Ω/□. Cell efficiency is 13.4%, withJ_(SC) of 30.0 mA/cm², V_(OC) of 0.593 V, and FF of 0.752. In spite ofthe very light n⁺ doping, the series resistance for this cell wasdetermined to be 0.70 Ω-cm², well within the 1 Ω-cm² limit desired.Attempts to use commercial Ferro 3347 Ag paste failed for n⁺ sheetresistances above 45 Ω/□ because of excessive series resistance.

The ability of screen-printed 151A and 151B Ag pastes to penetrate thesilicon nitride and make ohmic contact to a lightly-doped n⁺ layer usinga high-throughput belt furnace process demonstrates acommercially-viable material and process. Furthermore, measurements ofcontact resistance (current transfer length method) of 151A and 151Bcontacts through PECVD silicon nitride AR coatings to 60 Ω/□ n⁺ layersgave 3 mΩ-cm², equivalent to a series resistance of just 0.03 Ω-cm².Commercial Ferro 3347 Ag gave 500 mΩ-cm² contact resistance under thesame conditions, equivalent to a series resistance of 5 Ω-cm²,considerably above the 1 Ω-cm² limit. Measured bulk resistivity of the151A and 151B Ag contact material was quite low at 2 μΩ-cm, and thecontacts were readily solderable.

Considering the test results in total for the DuPont E89372-146Afritless paste and the DuPont E89372-151A and 151B fritted pastes, it isclear that a self-doping Ag paste has been realized for making ohmiccontact to n-type silicon. Such pastes, or a successors to them, areexpected to provide a practical, cost-effective material for makingself-doping negative electrodes to solar cells and other Si devices byalloying the dopant-coated Ag with Si. There is no obvious reason whysilver pastes incorporating coatings of a p-type dopant could not bemade as well. In theory, the process should also be applicable to othersubstrates such as germanium and silicon-germanium alloys.

The foregoing description of the preferred embodiments of the presentinvention is by way of example only, and other variations andmodifications of the above-described embodiments and methods arepossible in light of the foregoing teaching. The embodiments describedherein are not intended to be exhaustive or limiting. The presentinvention is limited only by the following claims.

What is claimed is:
 1. A method of manufacturing a contact, comprising:providing a semiconductor having a semiconductor surface; applying asilver layer to at least a portion of the semiconductor surface;applying a dopant to at-least a portion of the silver layer, the dopantbeing capable of doping the semiconductor; heating the semiconductorsurface, silver layer and dopant to a first temperature; maintaining thefirst temperature until at least a portion of the silver layer, aportion of the dopant and a portion of the semiconductor surface form amolten alloy; and cooling the molten alloy to a second temperature thatis below the first temperature such that at least a portion of thedopant contained in the molten alloy is incorporated into an epitaxialre-growth region of at least a portion of the semiconductor, the moltenalloy forms into a substantially solid first region containingsemiconductor atoms and dopant atoms and a substantially solid secondregion containing silver atoms and dopant atoms, and an ohmic electricalcontact is formed between at least a portion of the substantially solidsecond region and at least a portion of the epitaxial re-growth region;wherein applying a dopant is accomplished by applying liquid dopant. 2.A method of manufacturing a contact, comprising: providing asemiconductor having a semiconductor surface; applying a silver layer toat least a portion of the semiconductor surface; applying a dopant to atleast a portion of the silver layer, the dopant being capable of dopingthe semiconductor; heating the semiconductor surface, silver layer anddopant to a first temperature; maintaining the first temperature untilat least a portion of the silver layer, a portion of the dopant and aportion of the semiconductor surface form a molten alloy; and coolingthe molten alloy to a second temperature that is below the firsttemperature such that at least a portion of the dopant contained in themolten alloy is incorporated into an epitaxial re-growth region of atleast a portion of the semiconductor, the molten alloy forms into asubstantially solid first region containing semiconductor atoms anddopant atoms and a substantially solid second region-containing silveratoms and dopant atoms, and an ohmic electrical contact is formedbetween at least a portion of the substantially solid second region andat least a portion of the epitaxial re-growth region; wherein applying adopant is accomplished by applying an elemental coating.
 3. A method ofmanufacturing a semiconductor device, comprising: providing asemiconductor having first and second opposing surfaces; applying asilver layer to at least a portion of the first surface; applying adopant to at least a portion of the silver layer, the dopant beingcapable of doping the semiconductor; applying a metal layer to at leasta portion of the second surface; heating the first and second opposingsurfaces, the silver layer, the metal layer and the dopant to a firsttemperature; maintaining the first temperature until at least a portionof the silver layer, a portion of the dopant and a portion of thesemiconductor form a first molten alloy, and at least a portion of themetal layer and a portion of the semiconductor form a second moltenalloy; cooling the first and second molten alloys to a secondtemperature that.is below the first temperature such that at least aportion of the dopant contained in the first molten alloy isincorporated into at least a portion of a first epitaxial re-growthregion and at least a portion of the second molten alloy is incorporatedinto at least a portion of a second epitaxial re-growth region, suchthat the first molten alloy forms into a substantially solid firstregion in ohmic electrical contact with at least a portion of the firstepitaxial re-growth region, and the second molten alloy forms into asubstantially solid second region in ohmic electrical contact with atleast a portion of the second epitaxial re-growth region; and providingfirst electrical contact to the substantially solid first region, andproviding second electrical contact to the substantially solid secondregion.
 4. The method of claim 3, wherein the first molten alloycomprises proportions of the silver and the semiconductor first surface,the semiconductor proportion concentration being equal to or greaterthan the eutectic concentration, and the second molten alloy comprisesproportions of the metal and the semiconductor second surface, thesemiconductor proportion concentration being equal to or greater thanthe eutectic concentration.
 5. The method of claim 3, wherein thesemiconductor is selected from the group consisting of silicon,germanium, and silicon-germanium alloy.
 6. The method of claim 3,wherein the metal layer is aluminum.
 7. The method of claim 3, whereinthe dopant is selected from the group consisting of phosphorus, boron,antimony, arsenic, indium, aluminum and gallium.
 8. The method of claim3, wherein applying a dopant is accomplished by applying liquid dopant.9. The method of claim 3, wherein applying a dopant is accomplished byapplying an elemental coating.
 10. The method of claim 3, whereinthickness of the silver layer is in the range of 1 μm to 15 μm.
 11. Themethod of claim 3, wherein the first temperature is above eutectictemperature for the silver layer and the semiconductor.
 12. The methodof claim 3, wherein maintaining comprises maintaining the firsttemperature for a duration of at least one minute.
 13. The method ofclaim 3, further comprising: applying a second dopant to at least aportion of the metal layer, the second dopant being capable of dopingthe semiconductor; heating the second dopant to the first temperature;maintaining the first temperature until at least a portion of the metallayer, a portion of the second dopant and a portion of the semiconductorform the second molten alloy; and cooling the second molten alloy to thesecond temperature that is below the first temperature such that atleast a portion of the second dopant is incorporated into at least aportion of the second epitaxial re-growth region.
 14. The method ofclaim 13, wherein the second dopant is selected from the groupconsisting of phosphorus, boron, antimony, arsenic, indium, aluminum andgallium.
 15. The method of claim 13, wherein applying a second dopant isaccomplished by applying liquid dopant.
 16. The method of claim 13,wherein applying a second dopant is accomplished by applying anelemental coating.
 17. The method of claim 13 wherein the metal layer issilver.